The present invention relates to electrochemical cells, and in particular to bipolar plates for use in electrochemical cells.
Electrochemical cells are energy conversion devices, usually classified as either electrolysis cells or fuel cells. An electrolysis cell typically generates hydrogen by the electrolytic decomposition of water to produce hydrogen and oxygen gases, and functions as a fuel cell by electrochemically reacting hydrogen with oxygen to generate electricity.
Referring to FIG. 1, a partial section of a typical proton exchange membrane fuel cell 10 is detailed. In fuel cell 10, hydrogen gas 12 and reactant water 14 are introduced to a hydrogen electrode (anode) 16, while oxygen gas 18 is introduced to an oxygen electrode (cathode) 20. The hydrogen gas 12 for fuel cell operation can originate from a pure hydrogen source, methanol or other hydrogen source. Hydrogen gas electrochemically reacts at anode 16 to produce hydrogen ions (protons) and electrons, wherein the electrons flow from anode 16 through an electrically connected external load 22, and the protons migrate through a membrane 24 to cathode 20. At cathode 20, the protons and electrons react with the oxygen gas to form resultant water 14xe2x80x2, which additionally includes any reactant water 14 dragged through membrane 24 to cathode 20. The electrical potential across anode 16 and cathode 20 can be exploited to power an external load.
The same configuration as is shown in FIG. 1 for a fuel cell is conventionally employed for electrolysis cells. In a typical anode feed water electrolysis cell, process water is fed into a cell on the side of the oxygen electrode (in an electrolytic cell, the anode) to form oxygen gas, electrons, and protons. The electrolytic reaction is facilitated by the positive terminal of a power source electrically connected to the anode and the negative terminal of the power source connected to a hydrogen electrode (in an electrolytic cell, the cathode). The oxygen gas and a portion of the process water exit the cell, while protons and water migrate across the proton exchange membrane to the cathode where hydrogen gas is formed. In a cathode feed electrolysis cell, process water is fed on the hydrogen electrode, and a portion of the water migrates from the cathode across the membrane to the anode where protons and oxygen gas are formed. A portion of the process water exits the cell at the cathode side without passing through the membrane. The protons migrate across the membrane to the cathode where hydrogen gas is formed.
In certain arrangements, the electrochemical cells can be employed to both convert electricity into hydrogen, and hydrogen back into electricity as needed. Such systems are commonly referred to as regenerative fuel cell systems.
The typical electrochemical cell includes a number of individual cells arranged in a stack, with the working fluid directed through the cells via input and output conduits formed within the stack structure. The cells within the stack are sequentially arranged, each including a cathode, a proton exchange membrane, and an anode. In certain conventional arrangements, the anode, cathode, or both are gas diffusion electrodes that facilitate gas diffusion to the membrane. Each cathode/membrane/anode assembly (hereinafter xe2x80x9cmembrane electrode assemblyxe2x80x9d, or xe2x80x9cMEAxe2x80x9d) is typically supported on both sides by flow fields comprising screen packs or bipolar plates. Such flow fields facilitate fluid movement and membrane hydration and provide mechanical support for the MEA.
Fuel cells and, to a lesser extent electrolytic cells, have made extensive use of bipolar plates to provide fluids flow fields, sealing, and electrical continuity between cells in a cell stack. Use of bipolar plates can reduce cell stack size by replacing the separator plates, insulator plates, and at least two screen packs that are otherwise required with a single unit, the bipolar plate. The bipolar plate accordingly acts as both the hydrogen and oxygen flow fields and sealing member, thereby simplifying the stack and rendering it particularly useful in situations where space limitations are a controlling factor, for example in the automotive industry.
Conventional bipolar plates are typically molded or machined from carbon or composite blocks into the desired shape. However, in order to obtain the proper structural integrity and to enable the necessary machining, these components are typically very thick, on the order of greater than about 0.125 inches (about microns), especially in electrolytic cells, which often operate under high pressure differentials. Consequently, bipolar plates are often heavy and costly. Furthermore, the carbon plates are brittle, rendering these plates less useful for mobile electrochemical cell applications where shock and vibration are critical factors.
What is accordingly needed in the art is a ductile, light weight, less costly, readily produced bipolar plate assembly having high structural integrity.
A low-cost bipolar plate assembly for electrochemical reactors such as fuel cells and electrolysis cells comprises at least one foil sheet of an electrically conductive material having flow fields formed thereon.
An electrochemical cell stack using the bipolar plate accordingly comprises at least two membrane assemblies, each having an anode electrode and a cathode electrode disposed on opposite sides of an electrolyte membrane; a bipolar plate comprising at least one foil sheet of an electrically conductive material, wherein the sheet has a hydrogen flow field and an oxygen flow field to form separate hydrogen and oxygen flow passages, said bipolar plate being disposed between an anode electrode of one membrane assembly and a cathode electrode of another membrane assembly; and two end plates, one disposed on each end of the electrochemical cell adjacent to a first membrane assembly and a last membrane assembly.